In the field of continuous casting, in particular in the case of casting blooms and billets, it is known that one of the main problems relating to the quality of the finished product is the defect of rhomboidity. This defect in shape is characterized by the fact that the products, such as blooms or billets, especially for small formats cast at high speed, at the end of the solidification downstream of the casting machine do not have a profile exactly equal to the internal section of the crystallizer, but assume a rhomboidal shape which can cause problems in the subsequent rolling processes.
If the rhomboidity, measured as the difference between the diagonals of the section, is more than 6 mm, then the frequency of blockages in the first rolling stands increases.
This detect in shape is usually generated because of the lack of uniformity of heat exchange in the crystallizer, in particular in the zone immediately below the meniscus, which causes an uneven thickness of skin on the perimeter, both between one side and the other of the product and also along the same side. This unevenness depends on an asymmetrical deformation of the crystallizer, the entity of which depends on the intensity of the heat flow. Once generated, the deformation increases and cannot be recovered.
Moreover, because of the large exchange surface the edges are subjected to a significant heat flow, unlike the zone at 0-30 mm from the vertex of the edge. The latter zone therefore has a smaller thickness of local skin than that of the rest of the billet. A skin with a non-homogeneous thickness has weak points where the thickness is less and the formation of cracks under the skin is therefore frequent which can cause breakouts.
This problem is even more accentuated when free casting is done using oil as the lubricant. On the other hand, if lubrication powders are used during casting, the rhomboidity is less accentuated thanks to the insulating effect of the powders and their homogeneous distribution; however, the use of powders is more costly compared to the use of oil and is therefore uneconomic when commercial steels are being produced.
Moreover, the problem increases as the casting speed increases, which puts a limit on the maximum speeds obtainable and therefore on the productivity of the casting machine.
Rhomboidity is therefore a defect in shape due to uncontrolled conditions of adhesion between the liquid steel and internal walls of the crystallizer for a certain segment below the meniscus, that is at the moment when there is the greatest heat exchange and coinciding with the formation of the first skin, in which a non-uniform heat exchange occurs, and therefore a difference in thickness of the skin which is created along the perimeter of the billet as it solidifies.
In order to solve the problem of rhomboidity, document U.S. Pat. No. 6,024,162 proposes to make, on the internal walls of the crystallizer, a regular series of concavities, with the function of rendering the heat exchange more uniform in the critical zone just below the meniscus.
In particular, a zone comprised between 20 and 200 mm below the nominal level of the meniscus is identified in which regular rows of concavities are made, in the form of horizontal grooves, or small depressions of a circular, square or hexagonal shape.
This solution, even if it is an improvement, does not solve the problem on the one hand because the distribution of the concavities is not correlated to the development of the heat flow, and on the other hand because said concavities by themselves worsen the problem of the cracks under the skin and the breakouts in the more critical cases of free casting at high speed and using oil as a lubricant. Indeed, the concavities reduce the total heat flow exchanged between steel and ingot mold and therefore the average thickness of the skin at exit from the ingot mold.
Documents JP 8-206786 A and JP 11-000746 A describe crystallizers with grooves made substantially in the central part of the internal wall in order to reduce the heat exchange in the zone immediately below the meniscus. These solutions only partially reduce the problem, since in any case they create a lack of homogeneity of heat treatment on the entire perimeter of the billet.
From documents EP 1,792,676 A1, EP 1,795,281 A2, GB 2,177,331 A and JP 9-225593 A it is also known to make grooves on the external walls of a crystallizer. However, the grooves are provided either on the whole face of the crystallizer or in its central zone, and they are not therefore intended to homogenize the cooling treatment in combination with other means operating in correspondence with the zone of the meniscus.
The Applicant has devised, tested and embodied the present invention to solve the problems of the state of the art and to obtain other advantages as shown hereafter.